I think the main reason to consider the 70-200/4L IS is if keeping weight low is your primary concern. Personally, I prefer 70-300L for the zoom range and the more compact (retracted) size - the 70-300L fits vertically in many bags where the 70-200/4 would need to lay down and effectively take two lens spaces. The 70-300L is a better choice if you regularly use 300mm (better IQ and much more convenient than 70-200 + 1.4x).

I strongly agree here. I've owned both the IS and non IS version of the 70-200 twice, and I strongly prefer the 70-300L over them. The compact nature when retracted means more to me than the additional weight. My copy of the 70-300L is (in my opinion) sharper than any of the copies that I have owned of the 70-200 f/4 variants. It will be in my bag for a long time to come.

Tongue-in-cheek, joking only. I know what you meant, but the 70-300L makes precious little compromise against the 70-200 f4 for its benefit of extra flexibility.

Well, depends on the point of view. I can also say that the 70-200/4 IS makes little compromise in FL for its benefit of constant f/4, weight and IQ.

Since I often use it at 200/4 and have taken it to several continents, the 70-300 does not cut it for me. Neither does the 70-200/2.8 - too big and heavy. On the other hand, for local events, the 70-200II wold be my choice; for reach - the 70-300 would be better. Each lens has its own use.

OK,Given one would benefit from the extra range-So from what I've read the 70-300mm is as sharp as the 70-200mm f/4 IS through 70-200 at the minimum, perhaps even further out. Also, while the 70-200mm is f/4 constant, again my understanding is the 70-300mm is f/4 up to 200mm.

So, even if the 200-300mm range is ever so slightly less sharp and is also f/5.6, since the overlapping range between the two lenses is basically the same I am not seeing any disadvantage with this lens vs the 70-200 f/4 is... Thoughts?

because the 70-300L is not f/4 up to 200mm it is f/5 by 200mm (OTOH the 70-200 f/4 IS is a slightly slow f/4 at 200mm, more like f/4.25)

the 70-200 is also a bit lighter, you have that constant aperture

but yeah the 70-300L is good, I did end up selling my 70-200 f/4 IS after getting the 70-300L, not needed to swap on a TC is nice, that is SUCH a pain (plus, if you still swap on a TC, on the 70-300 it gets you to 420mm!!)

The 70-300 is not f4 to 200, it is 70-103mm f4; 104-154mm f4.5; 155-228mm f5; 229-300mm f5.6; so if that is important to you then it could make a difference, as could the 38% more weight for the longer lens.

Yes - so with the 70-200 you gain a faster aperture in most of that below 200mm range, and if you throw a 1.4x TC in your bag, the two are comparable at the long end of the zoom:

The 70-200 also benefits from a non extending design, and the combined weight of the 70-200 and 1.4x TC is just a bit lighter than the 70-300L (985g vs 1050g).

However, the 70-300 goes all the way out to 300mm, and if you're splitting hairs, it's slightly sharper there - plus, of course, there's no messing about with fitting/removing the TC to use all that range.

They are not comparable at the long end, the 70-300L is better unless you have outlier copies of either one or both. TDP either had a bad copy or messes up their 300mm tests. With my copies 70-300L was clearly better than 70-200 f/4 IS+1.4x TC III. The fact that TDP also had the 70-200 winning at 70mm f/4 also hints at something gone wrong. Most blogs have the 70-300L better at 280mm and at 70mm, wide open, so does photozone, so also implies Canon's own MTF charts.

Also at 280mm the 70-300L focuses 50% faster since you don't have the TC slow down.

OK,Given one would benefit from the extra range-So from what I've read the 70-300mm is as sharp as the 70-200mm f/4 IS through 70-200 at the minimum, [...]

TDP shows the 70-200 to be sharper at 200/4 than the 70-300 at 200/5.

Not on my tests, the 70-300L at 200mm f/5 was slightly sharper than the 70-200 f/4 IS at 200mm f/5 and noticeably sharper than the 70-200 f/4 IS at f/4, it also had less CA there (although the 70-300L has a lot more CA at 70mm, and yet it is also noticeably sharper at 70mm f/4 all the same; my 70-200 f/4 IS was definitely sharper at 135mm f/4.5 though).

My findings match Canon's MTF charts a lot more than TDP's do and they also match closer to what I've seen reported on a majority (but not all) blogs and to what photozone shows.

IMO, TDP either got the world's best 70-200 f/4 IS and/or very sub-par 70-300L and tamron 70-300 VC (all possible) or simply messed up their test or maybe tested them at like 4' to target or something weird.

I know that they are both L lenses and that all the L lenses are supposed to be sealed, but I don't think that all sealing is equal. It's a lot harder to seal a lens that is pumping air in and out than it is to seal a lens that is of constant length, so I would bet that the 200 is better sealed than the 300...... But on the other hand, how often do we use lenses in conditions where this would matter?

I just love my 70-300L. It is so versatile. A very capable travel lens In my judgement.

Totally agree with most here, had the 70-200 f4 IS for two weeks and switched to the 70-300L because I decided I needed the reach after switching to FF. No regrets here, very versatile lens, travels great and love the IQ.

I know that they are both L lenses and that all the L lenses are supposed to be sealed, but I don't think that all sealing is equal. It's a lot harder to seal a lens that is pumping air in and out than it is to seal a lens that is of constant length, so I would bet that the 200 is better sealed than the 300...... But on the other hand, how often do we use lenses in conditions where this would matter?

It rained every day last fall in VT and I used my 70-300L a lot and despite the extending front it managed to survive it no problems.

For a FF body I probably would pick the 70-300. For an APS-C the 70-200.

I have to wonder though, would f/4 be good enough for low light like Church ceremonies on APS-C? I haven't taken a camera to church so I would not know (though I will be this week!) I am thinking I could probably sneak away with lower ISO on the f/4, but will it be low enough...